The present invention relates to an optical device for use as an aid to driving, and is especially applicable to a motor vehicle. It makes use of technology known as "lidar" and is advantageously located in an indicator light or a headlamp of a vehicle.
The device uses at least one discharge lamp, of the kind filled with xenon or a gas of the same type, and generally utilizes technologies which have already been tested and are relatively widespread in the motor vehicle field, which make the application of the invention to this field possible.
The mounting of autonomous means for assisting navigation on board motor vehicles is regarded as an important developmental step with the aim of reducing the number of victims of traffic accidents.
Such autonomous means for assisting driving are for example described in DE 42 00 057, and are for example formed by information signalling means for the driver and/or means for controlling the speed of the vehicle.
Driving aids which are associated with the road infrastructure will also be involved in further development but are not the subject of this application.
A distinction can be made between active means and passive means when considering the autonomous means as a driving aid mounted on board the vehicle.
The latter generally call on optronic detection means, possibly with image formation and generally using bands placed in the near-infrared (0.8 .mu.m&lt;wavelength&lt;2 m) or far-infrared (bands II and III). A certain number of technologies are possible at present, but few are able to keep to the price constraints imposed in the motor vehicle field.
For this reason the present invention deals with active driving assistance means. These active means call on a wider range of techniques involving acoustic, optical or radar waves. Applied to the field of motor vehicles and to anti-collision devices, these technologies are often reduced to telemetry, even if one may still consider measuring the relative speed of the obstacle by the Doppler effect.
These techniques impose constraints on several criteria which are decisive:
performance, PA1 spatial requirement, PA1 costs, PA1 integration in the vehicle, PA1 synergy with pre-existing devices, etc. PA1 the semi-conductor laser diodes on the III-V InGuAsP/InP system and using electronic confinement structures with quantum wells. These components are still being developed and one should wait to see if their performance/cost relationship will considerably increase in the coming months or years; PA1 RAMAN lasers; this is the case of a YAG laser doped with neodymium pumping a container of methane under pressure (30-50 bars) and enabling a transfer of energy on the first Stokes order (1.06 .mu.m-&gt;1.54 .mu.m). Even if it is known how to make YAG Nd lasers in solid state, this solution nevertheless uses a pressurized gas with the latent risk of explosion in the event of an accident. This solution is expensive, and moreover its repetition frequency and lifetime is limited; PA1 optical parametric oscillators (OPO), which are certainly a path opening up very vast horizons to designers and users. Bringing into play advanced non-linear optical concepts, technologically they represent the most perfect solution with regard to performance. But the materials which are likely to enter their composition are still difficult to control, and they would cost too much if large quantities were produced; PA1 erbium lasers, which historically were the first to appear, which emit at 1.54 .mu.m and of which it may be thought that they are at the origin of many ocular safety standards, both military and civil. Their pumping efficiency is maximum with xenon lamps and today an attempt is still being made to improve them significantly with power diodes at 908 nm, the structure of which both as regards optical index and electronic confinement call on materials restricted to quantum wells. As line production could not produce sufficiently low prices, it has not yet been produced; furthermore the xenon lamps will still be the pumping means for the Er.sup.3+ ion which will be found for example in matrices of phosphate glass which are heat-resistant, possibly in the presence of the ions such as chromium, which, by energy transfer processes between electronic levels, contribute to a better pumping efficiency. PA1 at least one generator of a laser beam in a determined direction having a wavelength and with an energy or a power compatible with ocular safety; PA1 at least one discharge lamp of the type comprising a rare gas, such as xenon; PA1 a sample of an optical material doped with erbium or with at least one dopant for bestowing the sample of optical material with qualities of stimulated light emission; PA1 an optical system for concentrating the light flux generated by the discharge lamp onto the sample of optical material; PA1 an optical system for the output of the laser beam; PA1 and a laser beam receiver. PA1 to ensure the protection of the laser in the enclosure of the headlamp; PA1 no opening in the vehicle body, nor any additional adjustment precaution is to be envisaged by the manufacturer to direct the energy used for the telemetry, which is not the case for hyperfrequency-type radar; PA1 since there are two headlamps in a car, these lidars may possibly be doubled to ensure more reliability or the performance may be increased by time-division multiplexing of the pulses; PA1 emission may also take place from one headlamp and detection may occur in another by using a heterodyne arrangement, by which it is possible to avoid blinding the detector when the pulse is released. PA1 the presence of fog, PA1 the localization of targets such as the edges of the road or of nearby vehicles, PA1 the speed, the acceleration or the behavior either of the vehicle on board which the device is mounted, or of nearby vehicles, etc.
All weather utilization, particularly in fog or rain, will therefore be adopted for all radar devices.
Variations in the speed of sound, as a function of the atmospheric conditions, and also the greatest potential sensitivity to phonic pollution of ultrasound-based sensors, make them suitable for low performance solutions (reversing sonar for coaches, lorries, etc.).
The invention makes use of a technology known as "lidar". The spatial requirement and the directivity of the antenna of lidars is very advantageous owing to the fact that these result in greater simplicity of the signal processing. The technical problem resolved by the present invention lies in the adaption of this technology to the motor vehicle field so as to comply with the above-mentioned constraints.
More particularly, one of the essential aspects of the invention lies in the creation of a source for lidar which is compatible with these specific constraints.
From the possible candidates for the development of sources for lidars, the invention has sought solutions in the solid state for their greater reliability, their intrinsically lower production cost combined with a lower possession cost. The application of lidar, as an anti-collision device requires care to be taken to ensure ocular safety on leaving the pupil or laser antenna for evident reasons, which is a constraint that radar is totally free of.
In this connection, ocular damage is mainly of two types, i.e. corneal or retinal. Wave lengths greater than 2 .mu.m are generally greatly absorbed by water or the substances making up the cornea and are therefore to be proscribed wherever possible. Inversely, those less than roughly 1 .mu.m pass through the cornea and are focused on the retina, thus making the latter vulnerable to a too great energy or peak power. Luckily there is a fairly limited range where it is possible to use lasers without great damage to the eye and it is centered around 1.54 .mu.m.
To illustrate these considerations, the maximum energy densities permitted are respectively 5 .mu.J/cm.sup.2 and 1J/cm.sup.2 to at 1.06 .mu.m and 1.54 .mu.m on entering the ocular pupil, which represents a dynamic range of 5 to 6 orders of magnitude.
The technological solutions in the solid state enabling emissions with ocular safety are at present as follows:
The dimensions of the lasers having rare earth ions such as neodymium or erbium may vary greatly, with cavities in the order of one meter or more, just like one millimeter in the case of so-called micro-chip lasers. The latter are today actively developed in the U.S.A. for their excellent spectral and modal properties which enable heterodyne detection, and thus the measurement of the relative speed of an obstacle in relation to a vehicle.